(SPOILER ALERT) Been some time since I did that quest, but I think once You commit to "settle accounts" Your not going to finds the letters in the safes. You gather evidence (the notes in the safes) and take them to the head ranger in Mojave oupost and She
(Cassidy) thanks You for turning ALice and Gloria in.
Nah, if you can't get the evidence you just have to go the vengeance route, which you can always choose to do (even after going the peaceful route). You may have to go and talk to Cass again after settling accounts with Alice, even if she is with you when
you do it.